1. Field of the Invention
This invention is related in general to electrolytic processes and equipment for electroforming, electrolytic extraction and refining of metals. In particular, it describes an improved edge strip used to prevent deposition of material on edges of electrode plates during electrolytic processes.
2. Description of the Related Art
The principle of electrolysis has been utilized for decades to extract metals and other cations from electrolytic solutions. The extraction process is carried out by passing an electric current through an electrolyte solution of the metal of interest, such as copper, zinc, gold, silver, or lead. The metal is extracted by electrical deposition as a result of current flow between a large number of anode and cathode plates immersed in cells of a dedicated extraction tank house. In electro-refining, the anode is made of a material that is dissolved and therefore is lost during the process; in electrowinning, the electrode is more permanent. In both processes, the cathode is generally constructed of a metal alloy, such as titanium or copper alloys and various grades of stainless steel resistant to corrosive acid solutions. In the most efficient processes, each cathode consists of a thin sheet of metal of uniform thickness (2-4 mm) disposed vertically between parallel sheets of anodic material, so that an even current density is present throughout the surface of the cathode. A solution of metal-rich electrolyte and various other chemicals, as required to maintain an optimal rate of deposition, is circulated through the extraction cells; thus, as an electrical current is passed through the anodes, electrolyte and cathodes, a pure layer of electrolyte metal is electro-deposited on the cathode surface, which becomes plated by the process.
Similarly, to purify a metal in a refinery process using electro-deposition, an anode of impure metal is placed in an electrolytic solution of the same metal and subjected to an electric current passing through the anode, electrolyte and cathode of each cell. The anode goes into solution and the impurities drop to the bottom of the tank. The dissolved metal then follows the current flow and is deposited in pure form on the cathode, which typically consists of a starter sheet of stainless steel. When a certain amount of pure metal has been plated onto the starter sheet, the cathode is pulled out of the tank and stripped of the pure metal.
In both processes the pure metal deposit is grown to a specific thickness on the cathode during a predetermined length of time and then the cathode is removed from the cell. It is important that the layer of metal deposited be recovered in uniform shapes and thicknesses and that its grade be of the highest quality, so that it will adhere to the cathode blank during deposition and be easily removed by automated stripping equipment afterwards. The overall economy of the production process depends in part on the ability to mechanically strip the cathodes of the metal deposits at high throughputs and speeds without utilizing manual or physical intervention. To that end, the cathode blanks must have a surface finish that is resistant to the corrosive solution of the tank house and must be strong enough to withstand their continuous handling by automated machines without pitting or marking. Any degradation of the blank's finish causes the electro-deposited metal to bond with the cathode resulting in difficulty of removal and/or contamination of the deposited metal.
It is also very important that metal deposition be avoided along the edges of the electrodes to prevent the formation of a continuous layer of deposit between opposite sides of the plate which would complicate and delay the stripping process. Thus, in order to prevent electrolyte build-up along the double-sided edges of the starter sheet that would impede the automated separation of the product at the end of each cycle, these edges are masked with an insulating strip fastened to the electrode. Such edge strips are designed to tightly wrap around the edges of the starter sheet and prevent deposition of material past the line of contact between the strip and the starter sheet. In order to improve contact between them, several kinds of edge strips have been developed with different advantages best suited to specific applications.
For example, U.S. Pat. No. 4,406,769 to Berger (1983) discloses an edge protector consisting of a strip having an H-shaped cross-section so as to provide open slots on opposite sides. One slot is defined between a pair of parallel jaws and is adapted for receiving the edge of an electrode; the other slot is substantially semicircular and is adapted to receive a tubular member in compression, so that its insertion results in a leveraged narrowing of the first slot and a corresponding tight frictional connection between the edge strip and the cathode.
In U.S. Pat. No. 4,776,928 (1988), Perlich describes a coextruded structure for an edge protector consisting of a rigid U-shaped member having parallel jaws that define a slot for receiving the edge of an electrode and a pair of resilient lips attached to the ends of the jaws to press tightly against the electrode edge, thereby impeding penetration of electrolyte. This patent first disclosed the concept of using dual-durometer coextruded members to improve gripping of the edge strip to the electrode surface.
In U.S. Pat. No. 5,314,600 (1994), Webb et al. introduced the concept of including a longitudinal groove within the edge slot for accommodating and engage transverse pins protruding from the electrode. This edge protector also includes expansion channels to facilitate the insertion of the electrode's edge into the protector's slot.
While amounting to substantial improvements over the prior art, these edge protectors retain some features that from time to time may still cause problems. Edge strips need to be sufficiently rigid to retain their shape over severe temperature cycles and maintain continuity of contact with opposite surfaces of the starter sheet's double-sided edges. At the same time, the compressive force exerted by the strip on the edge depends on the resilience of the strip's material, which is critical to ensure a sufficient degree of compression on the edge and prevent penetration of electrolyte solution during the deposition process. If the material is too rigid, the edge strip's performance becomes very dependent on a perfect fit of the starter sheet within the strip's edge slot; if too resilient, the strip may more easily conform to variations in smoothness and thickness in the starter sheet but it may also be easily deformed by shocks and buckling forces that result in electrolyte solution penetration and material deposition within the strips boundary.
Berger improved the clamping ability of edge strips by providing two opposing slots along the length of the strip: an edge slot for receiving the starter sheet edge and a lever slot for receiving an elongate expansion member. This leveraged configuration maintains compression of the strip on the starter sheet by the action of the expansion member that is forced into the lever slot opposing the edge slot along the length of the starter sheet. Obviously, the leveraging effect of the expansion member is affected by the rigidity of the strip's material, a greater rigidity of the hinge component limiting the leveraging action transferred to the edge slot and the corresponding adherence of the strip tips to the starter sheet surface. On the other hand, sufficient rigidity in the lever-arm material is necessary to transfer the compressive force to the tip of the edge slot. The rigidity of the material utilized for this unibody configuration tends to cause longitudinal cracks in the hinge component, especially when the edge strip is mounted in cold weather. Therefore, in order to minimize this problem, the strips are commonly heated prior to installation in cold temperatures, with produces substantial inefficiency and related expense.
Perlich partially solved this problem by providing a dual-durometer edge that combines the longitudinal rigidity of a standard edge protector with the greater resilience of a coextruded converging lip made of softer material. This type of strip improves contact between the strip tips and the surface of the starter sheet because of the compressive force exerted by the converging resilient material at the tip of the edge slot, thereby ensuring contact at the tip even without a leveraging action, but its performance remains sensitive to the resilience of the soft lips.
Therefore, there still exists a need for an improved electrode edge protector. The present invention provides a simple method of construction for producing such an improved device.